sb head question

There are probably a couple dozen threads you could find in the search function that would detail exactly what cuda just mentioned.

The advantage of going from the stock small valve/small port 68 318 heads would likely be about 25-30hp if not more as long as the donor heads are in good shape.

MP has a part number that's supposed to be a drop in replacement for the flat tappet to magnum head, but like cuda also mentioned, you'll need to swap to an AMC oil through type lifter...unless your '68 318 has had a cam change in the past 10-15 years with a new off the shelf cam/lifter kit, it'll have non-oiling lifters.

In short, you need:

magnum heads, complete with rocker gear
magnum head bolts-MP
magnum style intake manifold-available from Edelbrock, MP, and Professional Products
correct length pushrods-not sure on the length, search and ye shall find
ensure your lifters have oiling provision--easiest to buy a set of AMC lifters...
all appropriate gaskets
*this is only my opinion, but spend the $12 and buy a new set of intake bolts from Summit/MP...cheap insurance, you don't want to risk installing a new intake with corroded old hardware

many folks on here run the LA style valve covers, you just have to fiddle with the gaskets a little, or just use the magnum covers.